Applications in which a flowable material is to be relatively uniformly applied onto a predetermined area or surface are numerous, varied, and constantly growing. For example, steel products require a protective coating of rust prohibitive oil following the manufacturing process to protect the finished products during shipping, storage, processing and the like. Similarly, products such as galvanized steel, fabrics, food products, and other materials also often require application of a predetermined coating or treatment of liquid or other flowable material for a variety of reasons. While conventional spraying techniques, physical application, dipping, wiping, soaking and other procedures have been implemented with varying degrees of frequency and success, efficiency and reliability of quality and coverage is most often of paramount importance in modern application environments.
U.S. Pat. No. 4,749,125, which issued to Escallon et al., pertains to a method and apparatus for electrically charging and dispensing fluids and the like to allegedly overcome the problems of prior art dispensing orifices and mechanical means for dispensing fluids. Particularly, Escallon et al. describes the previous use of small dispensing orifices, mechanical means such as spinning disks, or aerodynamic means for finely dividing fluid into droplets. Such prior techniques and devices suffered from problems of clogging, non-uniformity of application, and inefficiency of energy use and application volume. This patent emphasizes the importance of controlling material droplet size and the overall uniformity of dispensing in most applications.
The Escallon et al. nozzle is described as including a fluid reservoir in a housing which defines a chamber having a resiliently compressible elongated slot at its tip. A shim is provided in the chamber slot, and the thickness of the shim and the compressing force on the chamber serve to define the size and shape of the slot for dispensing. The shim and the fluid are electrically connected to high voltage, which causes the fluid meniscus which forms at the slot to be dispensed from the nozzle as charged droplets. Escallon et al. contemplates voltages of between about 10 and 50 kilovolts for dispensing fluids in a viscosity range of between about 1 and 20,000 cps, and teaches that precision selection of the shim determines the flow characteristics of the dispensed fluid dependent on the fluid pressure within the chamber. This patent also teaches that the distal edge of the shim must have a discontinuous geometry to control the rate of flow through the nozzle.
It has been observed, however, that nozzles made in accordance with the teachings of Escallon et al. often encounter problems in providing an application spray of predetermined, uniform consistency for coating of material at a predetermined rate per volume of area. Particularly, there is a clear lack of ability to carefully control the volume of material coated on the target area, and a lack of control of the uniformity of such application. Additionally, in many applications where electrostatic dispensing is useful, the application equipment must be reliable and easy to clean and maintain. For example, in applications involving food or other edible products, the equipment must be maintainable in clean and healthful conditions to meet standards of quality under applicable food and health laws and the like. In manufacturing applications, it is often required to alternately change between flowable materials to be coated, and time required for such changeover is critical to productivity and profitability. Moreover, to obtain acceptable uniformity of material dispensing, material dispensing flow rates and uniformity of dispensing across the nozzle must be reliable and continuous. The prior art devices could not deliver these requirements.
It should also be noted that due to the relatively high voltage necessary to properly incorporate electrostatic deposition of flowable materials, adequate support of the high voltage components is critical. The voltage is constantly seeking the path of least resistance, and the device will be ineffective for dispensing procedures if such voltage finds an alternate path to ground. In addition to the problems discussed above, electrostatic dispensing nozzles available in the industry heretofore did not provide adequate support for the high voltage power, and were relatively unreliable and difficult to maintain on line as a result of the relatively complex support structures required to accommodate a plurality of nozzles arranged in series to provide a predetermined dispensing width.
Particularly, the nozzles suffered from leakage of flowable material and down-time caused by nozzle grounding and cleanup requirements. Such electrostatic dispensing nozzles were available in predetermined widths of about 6 inches (about 15.2 cm) and about 3 inches (about 7.6 cm), and application widths for particular coating procedures were obtained by side-by-side alignment of a plurality of such nozzles. The smaller nozzles were recently developed in response to overspray and underspray problems generally encountered when the width of the target to be coated was less than or greater than the width of the aligned nozzles. Additionally, at the interface of each adjacent nozzle, there was often a discontinuity in the application of the flowable material, causing corresponding discontinuities in the overall uniformity of material application.
Additional problems arose where electrostatic dispensing was required from below a product or target, wherein electrostatic dispensing nozzles were required to "shoot up" in order to coat a target from below. Particularly, in addition to the problems discussed above, nozzles available heretofore simply could not adequately overcome the additional problems imposed by gravity, and failed to reliably provide a uniform application of flowable material from below the target at a controlled application rate.